More than 2,000 flights cancelled, hundreds of thousands of passengers delayed or stranded, and airlines facing compensation payments in the millions.
How did a fault in the UK’s air traffic control system that was fixed within four hours cause such prolonged chaos?
It was a bank holiday Monday – a classic “big travel day”.
Some families jetted from UK airports for the final days of summer, while others flew back ahead of the return to school for most children.
Early news of delays at some airports did not cause much panic – they happen every day and, more often than not, are rectified almost immediately.
But a message from Scottish airline Loganair posted on social media site X, formerly Twitter, shortly before 12:00 BST suggested a bigger problem.
The airline said there was a network-wide failure of UK air traffic control computer systems.
Within half an hour, the problem was confirmed by Britain’s National Air Traffic Services (Nats). It said in a statement it was experiencing “technical issues” and was restricting air traffic flow for safety reasons. It did not say how long it would take its engineers to find and fix the problem.
All flights could land safely. carlsbadstation.com was not a matter of life and death – but of frustration and expense, on a scale rarely seen in any setting.
Some planes already flying back to the UK turned around and landed. Others simply stayed on the ground.
Sports presenter Gabby Logan, in Budapest after the World Athletics Championships, said she was on a plane when passengers were told they could be held up for 12 hours.
Ex-air traffic controller describes what happened
What exactly had gone wrong was not being fully explained by Nats.
But Michele Robson – who worked as an air traffic controller, air traffic supervisor and safety manager from 1992 to 2016 at the London Area Control Centre in Swanwick – gave the BBC insider insight into the systems.
The failure in the air traffic planning system would have started about 08:30 BST, she says – several hours before Loganair’s tweet.
A back-up system could have operated normally for another four hours, but those working on a fix would have been fully aware they were against the clock, and then later realised they did not have enough time.
“They decided to go manual. This means the flight planning assistants and ATC assistants input the flight data manually.
“This is something they do all the time but there is suddenly a lot more pressure on them,” Michele says. It would have been a tense few minutes for the air traffic controllers when they went to “manual” – but it is a situation they train for.
She recalls reverting to a manual system just once in her career: “You have to make more phone calls but it’s not the scene of panic. Whenever people came into our workspace they always said they were surprised by how quiet it is.”
Aviation data firm Cirium was busy calculating the size of the problem. It said there were 3,049 flights scheduled to depart from the UK on Monday and a further 3,054 due to arrive.
Airlines and airports were by now issuing the standard advice for passengers to check the latest status of flights.
But there was still no indication of how long the problem would last.
Delays and uncertainties were having a big impact on some. Serena Hamilton, from Cookstown, County Tyrone, was meant to fly to Newcastle for a hospital check-up following a heart transplant last year – but realised she had no hope of getting there in time.
Airlines are hampered in how quickly they can get things back to normal during long delays because safety laws limit the number of hours staff can work. And if planes get stuck in one place, they may also miss their next scheduled flight from another country.
Companies were soon warning of outright cancellations. Passengers were scrambling to get on the next scheduled flight, either with the same airline or a competitor – all without knowing whether that next flight would also get stuck.